Things I think are cool in the first MESSENGER image of Mercury

OK, the baby's in bed and I have time to examine the new face of Mercury. To review: this is the first image returned from MESSENGER following its first flyby of Mercury, on Monday, January 14, showing a face of Mercury not seen by Mariner 10 (though it has been observed using radio telescopes). Originally, more images were expected today, but because of conflicts with the Deep Space Network having to do with anomalies on the Ulysses solar observing mission and elsewhere, MESSENGER lost some of its expected communications time. Because the image was received so late in the day Tuesday, when much of the world was already asleep, NASA considered holding its release until Wednesday morning; but it seems that the media have been so impatient to see it that, in an unusual move, they released it at 8 pm Eastern time today.

OK. Here's the image.

NASA / JHUAPL / CIW

First look at the unseen face of Mercury

This image was taken by the MESSENGER spacecraft at 20:25 UTC on January 14, 2008, about 80 minutes after its closest approach to Mercury. Most of the face of Mercury visible in this image was not seen by Mariner 10.

Most people's first reactions to images of Mercury are that they look an awful lot like the Moon. And that's no accident; they are both airless bodies with rocky surfaces that have experienced little surficial geologic activity in the past three or four billion years other than the formation of numerous impact craters across their surfaces. Both are speckled with a few relatively fresh, bright, rayed craters, which adds to the apparent similarity. However, there are important differences between Mercury and the Moon, differences that hint at very different origin and evolution for the two bodies. For comparison's sake, here's a nice image of the lunar farside, at similar phase to MESSENGER's Mercury photo. (This one is from the ill-fated Nozomi, Japan's mission to Mars.)

ISAS / JAXA / Ted Stryk

The Moon from Nozomi

Nozomi captured this view of the lunar farside using its Mars Imaging Camera (MIC) on September 24, 1998.

Okay, so, what are some differences? The most obvious difference between Mercury and the Moon is that the Moon has bright highlands, filled in in places with dark basaltic lavas forming the lunar seas or "maria;" Mercury has nothing at all like this. Instead, across the globe of Mercury, even in this monochrome image, you see hints of subtle variations in the brightness of the surface, hints that speak of a complex geologic history that may change from one locale to another. I'm particularly taken with the strange dark-rimmed craters in the Mercury image -- one way of explaining those is to say that there's layers under Mercury's surface of rocks with different composition, and the impacts that formed those craters dug into different layers, exposing different rock types concentrically.

The apparent lack of very large basins on this face of Mercury surprises me too. Part of that has to do with how it's lit. The new MESSENGER image includes the entirety of the Caloris basin, only half of which was visible in Mariner 10 images, but despite being one of the biggest impact basins to be found anywhere in the solar system it is astonishingly difficult to see in the MESSENGER view because the Sun is nearly directly above it. Here's Mariner 10's view:

NASA/JPL/Mark Robinson

Mercury's Caloris Basin

The Caloris Basin is one of the largest basins in the solar system at approximately 1,300 kilometers (800 miles) in diameter. Only half of it was seen by Mariner 10 as it sped past Mercury in 1974. Source Color: Greyscale. Scale: 1000.00 meters per pixel. Created: 29 March 1974.

And here is my best effort (admittedly, done rather quickly late at night) to find Caloris in the MESSENGER photo. Finding the rim of Caloris, which seems so prominent in Mariner 10 images, is really, really hard to do; my dashed line is barely more than a guess. I do find it interesting that there is a big cluster of those dark-rimmed craters associated with Caloris.

NASA / JPL / JHUAPL / questionable mapping by Emily Lakdawalla

Whither Caloris?

The Caloris basin, such a prominent feature on the terminator in Mariner 10's images of Mercury, is nearly impossible to find in the first MESSENGER views, which are lit by a much higher sun.

In general, topography on Mercury seems really subtle, which is why it's so great that MESSENGER carries a laser altimeter. Once the mission is over, we should have a very nice map of Mercury's topography; I predict that there will be many more large impact basins discovered in the topography that are pretty much invisible in the images.

One place it's easy to find topography is on the terminator, a term that still makes me giggle; for those of you not in the know, the "terminator" is the day-night boundary, where the sun slants onto Mercury's surface at a low angle, casting long shadows. Caloris was discovered in Mariner 10 images because it sat on the terminator, so the long dawn shadows accentuated the subtle topography along the multiple rims. I don't see any new large basins on the terminator in this photo, but I see something else that, as a structural geologist, I find just as exciting: rupes. "Rupes," the Latin word for "cliff" or "escarpment," represent areas where the crust of Mercury (or another world -- there are rupes on the Moon and on Miranda, for instance) has deformed under some kind of compressional stress. Here's the most prominent rupes I see. I've blown this cutout up by a factor of two:

NASA / JHUAPL / CIW

New rupes found on Mercury

On the terminator (day-night boundary) in the first wide-angle shot of Mercury returned from MESSENGER's first flyby are several steep scarps that cut across craters, like the one at the center of this photo. Such scarps are called "rupes" on Mercury and are places where the planet's crust has deformed, buckled, under compression.

You find these rupes across Mercury, cutting across craters and shortening them (in fact, you can make very nice quantitative measurements on how much shortening has happened by assuming the craters were initially circular and measuring by how much crater the rupes have cut out) but there aren't any obvious tensional features. On Earth, compressive features like mountain belts are sort of balanced out by extensional features like rift valleys and ocean basins. Earth's lithosphere splits, moves, collides, and buckles, but when you add up all the plate motions, you get as much lithosphere created by extension as you see destroyed by compression. On Mercury, it seems, we mostly find these compressional features, which (unless we just haven't spotted the extensional features yet) means that the whole lithosphere of the planet has shrunk. Which can only happen if the whole diameter of the planet has shrunk. Now, this is fairly simple to explain if you accept that Mercury has cooled off a lot since it formed -- most matter contracts as it cools, and if Mercury's core got smaller by cooling then its outer layers had to shrink too. But Mercury has this pesky magnetic field that indicates it still has a molten core, which throws a wrench into the works of that whole planet-cooling theory. There aren't any answers to be had in this one image, but hopefully MESSENGER will eventually return data that will help us solve this conundrum.

I'm getting sleepy so I'd better call it a night. The last time I talked to someone on MESSENGER it sounded like they probably got more than just this image in this evening's communications session, so I'm certain we'll see more images released tomorrow.

I will mention one last little item that only a true geek will love. The caption that the MESSENGER team released with the image did not include the exact time at which it was taken. However, you can figure out the time it was taken based upon its filename, "EW0108829708G." The filenames on the released Mercury images have the same form as the ones I've seen for MESSENGER images already released to the Planetary Data System: the filename has 13 characters, of the form "Eannnnnnnnnnp." What "E" means I'm not sure -- every file name begins with that, perhaps it indicates data from the camera instrument as opposed to any other instrument -- but the rest I can interpret. The character in position "a" is "N" for the narrow-angle camera and "W" for the wide-angle camera. The 10-digit number "nnnnnnnnnn" measures time, in seconds, counted up on the spacecraft clock. And the character in position "p" designates which color filter was used for the image (it's always "M" for narrow-angle camera images, where there are no color filters, but can be letters A through K for the 11-filter wide-angle camera; in this case, it's the G filter, a narrowband one at a wavelength of 750 nanometers, just over the edge of red into the infrared, which is, incidentally, very similar to the wavelength of the filter most often used by the rovers' Pancams for imaging rocks on Mars). Since I know the times of earlier images released by the MESSENGER team, for instance the one named "EN0108529741M" that was taken on January 11 at 09:06, I can figure out from the name of this file that it was taken 299,967 seconds later, which happens to be on January 14 at 20:25. Piece of cake.